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In quantum mechanics we learn that the behavior of the very smallest
objects (like electrons, for example) is very unlike the behavior of
everyday things like baseballs. When we throw a baseball at a wall, we
can predict where it will be during its flight, where it will hit the
wall, how it will bounce, and what it will do afterward.

When we
fire an electron at a plate with two closely spaced slits in it, and
detect the electron on a screen behind these slits, the behavior of the
electron is the same as that of a wave in that it can actually go
though both holes at once. This may seem odd, but its true. If we
repeat this experiment lots of times with lots of electrons, we see
that some positions on the screen will have been hit by many electrons
and some will have been hit by none. The observed "interference
pattern" for these electrons is evidence of their dual wave-particle
nature, and is well described by thinking of each electron as a
superposition of two "states", one that goes through one slit, one that
goes through the other.

To add to this already mysterious
behavior, this interference will only happen if both possible paths
that the electron can take are not distinguishable. In other words, if
we could somehow tell which slit the electron went through each time,
we would no longer get the interference. The act of making a
measurement of the electrons path fundamentally changes the outcome of
the experiment.

Mats

(published on 10/22/2007)

Follow-Up #1: quantum

Q:

I understand that Quantum Physics is complicated and I'm just trying to wrap my head around this answer. So a couple of questions.
1. Is this done in a complete vacuum?
2. If not how do you know you aren't striking another electron and the two electrons together are passing through each slit?- TwoNames (age 32)Kamiah, ID, USA

A:

I'm not sure it's so much "complicated" as very unfamiliar.

Anyway:

1. It's nearly a complete vacuum. Collisions with gas make the paths distinguishable and destroy the interference pattern.

2. The pattern only appears for good vacuums. It remains good even when the electron beam is so weak that there are almost never two electrons present at the same time.

Mike W.

(published on 11/25/2009)

Follow-Up #2: observation and light

Q:

By observation, does that mean using light? Because, I could understand how "observation" would muddle things up if locating the electron at a point in time required possibly interfering with its path.
...I'm just wondering if the literal Copenhagen interpretation can be side stepped.- Devon (age 23)Lansing

A:

Our usual stories about observation involve light, but it isn't essential. Anything that leaves some sort of external record which differs depending on which path was followed will do. Mike W.

(published on 12/01/2009)

Follow-Up #3: measurement and consciousness?

Q:

Are electrons conscious i.e. do they know that they are being observed? Secondly, do they change their behaviour when someone tries to watch it?- Indrajit KuriNew Delhi, India

A:

It would be extremely surprising if anything as simple as an electron could have any sort of consciousness. However, when someone tries to watch an electron, they usually do something to the electron to make its behavior more evident. That something (e.g. shining a light on it) changes the electron's behavior.

Mike W.

(published on 08/20/2010)

Follow-Up #4: free-will?

Q:

As a follow up to the previous answer, I'd like to share something I found interesting. If one defines free will as something like "non-deterministic", one can prove from three simple axioms that if you wish to claim we (experimenters) have "free will", then we must conclude electrons have "free will" as well.
http://en.wikipedia.org/wiki/Free_will_theorem
It is an interesting way to demonstrate the "measurement problem" in quantum mechanics, for in the traditional Copenhagen approach the evolution of a system is completely deterministic except for a measurement. If it turns out all of physics can be explained with the appropriate choice of lagrangian, can we really have the freewill to choose a random measurement?- Kevin M (age 29)Urbana, IL, USA

A:

Thanks for the interesting link. Just to expand on a point mentioned in passing in that article, there is a strong distinction between the indeterminacy described by the theorem and the traditional concept of free will. Readers should be forewarned that what follows somewhat spills over the edge of physics into philosophy.

There are serious reasons (including the violation of the Bell Inequalities) to conclude that the sort of events described by quantum mechanics are "free" in the sense that no prior fact about the universe can tell us which outcome we will observe. That doesn't mean that the necessary determining facts are hard to find; it means they didn't exist.

On the other hand, when we think of "free will" we have the sense that there was some prior "will" which determined what we chose to do. However, the existence of any such will would violate the theorems as much as any other determining variable.

Thus since the peculiar randomness of quantum events undermines the deterministic picture of the world it could be said to indicate a sort of "freedom", but not anything resembling traditional "free will."

Mike W.

(published on 08/24/2010)

Follow-Up #5: confusion between the uncertainty principle and the observer effect

Q:

There's a lot of confusion between the uncertainty principle and the observer effect, leading to the new age, nonsensical claim that we can willfully create the world around us by altering our thoughts. So, to be clear (because there's a lot of conflicting info out there), when we talk about "observing" an electron and thereby changing its state, we're talking about using equipment to measure it, not simply observing with the naked eye, right?- Ian (age 29)California

A:

Right, we have no indication at all that interaction with conscious beings (e.g. us) does something different than interaction with any other large object in which some record is left of the results. Of course, the only events we are aware of are those of which we are aware, but we can leave that worry for the philosophers. At any rate, the structure of quantum mechanics, in particular its violation of the Bell Inequalities, would run into big trouble if the random outcomes of quantum events were influenced by any local variable, including human will.

So you're right on all your key points. Nevertheless, there is a relation between the "observer effect" and the uncertainty principle. Mathematics requires that any wave, including purely classical ones, have a "spread" relation: ΔkΔx >= 1/2. That says that the spread (Δ) in the wavevector (k, sort of the inverse of the wavelength) times the spread in position (x) is greater than or equal to 1/2. The classical wave simply must have spreads in both these attributes, just as you can easily picture for water waves. We don't call this "uncertainty" or make a philosophical fuss about it because, as you can see by eye, the spreads in position and wavevector are real, persistent things.What's weird about quantum waves, though, is that when they're "observed" or "measured" we don't see the full spread that was there in the wave. If you set up apparatus to measure x, you see an output that has a very narrow range of x, even if the input is a big spread of x. Likewise if you measure k, the output has a narrow range of k. It's as if the wavefunction "collapsed" in a way guided by the type of measurement made. As to which particular little range of, say, x it collapses to, there's just a probability rule. The detailed result is purely random, not guided by any prior content of the universe. That's what converts the quantum spread into quantum uncertainty.

So people have good reason to link these effects and to be very puzzled by the whole business. As is common in cases of confusion, some people use the occasion to claim to be the center of the universe and to have magical powers. Other people buy it.

Mike W., Shalin, Samson

(published on 12/13/2012)

Follow-Up #6: Quantum for the non-scientist

Q:

I have a friend (not a physicist) who reads unscientific articles on things like schroedinger's cat and superposition and the 2-slit experiment phenomena, and as a result comes to believe that quantum mechanics undermines the logical rules of our universe because you can say things like "the particle is in two places at once" and applies it to more of reality than it should be. I feel like I have a pretty good understanding that his problem lies somewhere in a bad understanding of the uncertainty principle and/or observer effect (similar to the I can make things happen that I want because of the observer effect issue discussed in the last comment, but not quite the same). I'm having a hard time articulating though, I was wondering if you could help?- William (age 26)Columbus, OH, USA

A:

Hi William-

We'd love to help on that, but it would be much more effective if your friend could follow up with some specifics in his or her own words. We've written so much on the topic, generally searchable via the phrase "Bell Inequality", that we have little to add. I think it's fair to say that both quantum mechanics and relativity shake up a lot of basic ideas about the universe. That doesn't mean that they leave nothing of ordinary logic. just that what's left is modified- especially in the case of quantum mechanics.

Mike W.

(published on 09/11/2013)

Follow-Up #7: Unconscious observers

Q:

Has the double-slit experiment ever been done with animals, birds, insects or other creatures "watching"/"not watching", AND also without any man-made recording devices turned on ("watching")? If so, what were the results of having ONLY non-human conscious beings "watching"/"not watching", who presumably don't even know that they are "watching"/"not watching" anything?- Marshall Curtis (age 59)Bellevue WA USA

A:

Any measurement process that has a permanent effect on the system of interest causes the collapse of the wavefunction to a particular state, regardless of whether/how the results are interpreted by a human being. As long as the measurement device is on and recording, the state will be altered. Take temperature measurements, as an analogy. Although one tends to neglect, thermometers have a non-zero heat capacity in real life, therefore when you dip one into warm water, its temperature will be slightly decreased. Dip a thermometer in hot water, wait 1 minute, take it out and dip another one. The second time you measure will give you a lower measurement, compared to the case if you totally omitted the first measurement. This will happen whether you look at the mercury level or not, your dog watches the mercury level or not. What causes a change is the thermometer itself, not existence of a conscious mind watching it. In quantum case, the measurement device causes a change just because it records the state (not because of a side effect).

Tunc

p.s. The vast majority of modern experiments of this type use automatic fast recording of the "which-way" data that are measured. There's no direct observation by people until the overall results are done. As Tunc wrote, it's the recording which sets up some physical difference in the outside world that depends on which-way. That breaks the interference between the paths. Mike W.

(published on 06/13/2015)

Follow-Up #8: Bell's Inequalities and free will

Q:

I admire this website and it is very interesting. May I post a question regarding an answer that was given to a question concerning "free will" in the electron double slit experiment? The answer was #4 posted on 8/20/2010. by Mike W. This question is for Mike. Can you tell me please what you mean by "strong distinction between the indeterminacy described by the theorem and the traditional concept of free will". On the contrary it seems like Bell's theorem not only allows but demands "free will". By definition free will implies non determinate. If I had no free will to choose the experiment choice then it can be said that the outcome was predetermined since it was your action that caused the electron to take the particle or wave path. See. MIT News Feb 20,2014. "Closing the "Free Will" Loophole. It's the reverse of what you would think. Lack of free will on the part of the experimenter is the problem because then the particle detector setting can "conspire" to influence the outcome. It is considered the third and last loophole in order to validate Bell's Theorem. Please comment if possible and by the way I enjoy the Ask the Van ProgramSincerelyJulius Mazzarella. ( Retired International Paper 2006 )- Julius Mazzarella (age 63)Middletown, Ohio U.S.A.

A:

You're quite right that a conspiracy involving both the "random" or "free will" detector settings and the supposedly entangled test particles could produce the observed violations of Bell's Inequalities. It would have to be quite a conspiracy, in which apparently random events in two remote detectors and in some particle source were all pre-choreographed by some entity determined to mess with our minds. So the conclusion is that either quantum events have truly random outcomes, in principle not predictable from any prior facts about the universe, or absolutely everything is exactly determined in an amazing pseudo-random conspiracy. Either view is logically consistent, but most of us accept the random version.

As for how this randomness differs from the traditional concept of "free will", the issue does not concern "free". It concerns "will". Something that comes from no prior fact about the universe is not what we usually think of as "will", which flows from a prior mental state. If one thinks of "will" as a non-local variable, then one could say that it might be involved. That doesn't seem to fit well with what we know happens to thoughts in response to various direct physical and chemical events in the brain, which is as local as any other physical object.

p.s. That MIT article proposes using distant quasars to get the random detector settings. The idea is that it's really hard to imagine them both being in on the same extremely detailed conspiracy. I heard that a fellow at the South Pole who started worrying about these issues when he took a class on them from me had an even better idea along the same lines. Use the cosmic microwave background radiation from opposite directions. Alas, at least in our standard picture, the homogeneity of the CMB is attributed to inflationary causal connections between every part of the visible universe. So even these experiments wouldn't logically rule out the conspiracy idea. They would certainly help dramatize its absurdity.

Mike W.

(published on 07/05/2015)

Follow-Up #9: free will and quantum mechanics

Q:

A followup question/s for Mike W. ( Thank you for your time to shed some light on this. My problem may have been one of semantics which you helped shake loose. Don't worry I will not turn this into a philosophical conundrum.) My first follow up question is concerning the double slit experiment. This is where to me it seems "free will" and consciousness is an issue since the experiment always has the same outcome DEPENDING on which experiment you choose. If you can claim conscious free will made the choice then the outcome of the experiment really does depend on which experiment is "freely" chosen and QM survives otherwise a deterministic outcome can be argued. From the experiment's "perspective" this choice is random since you have the choice to break any predetermined outcome. I agree with you the entanglement issue does not require free will. The outcome will always be 50/50 at the "head" of the experiment and oppositely correlated at the "tail". ( However far apart the "tail" is.!amazing ) BUT the entanglement experiment to me does not EXCLUDE local free will at the start or "head" of the experiment. Free will only activates the experiment, the experiment will do whatever it wants. By the way you wouldn't know any really good beginners books on this topic would you? Sincerely Julius Mazzarella- Julius Mazzarella (age 63)Middletown

A:

There's some nice discussion of these topics (and many others) in Scott Aaronson's book "Quantum Computing Since Democritus".

Again, there's certainly nothing in these Bell violation experiments to discourage the idea that some of our choices are "free". It's hard to see, however, how those free quantum choices can be the result of anything we'd call "will".

Mike W.

(published on 07/07/2015)

Follow-Up #10: what collapses wavefunction?

Q:

In one of the earlier answers you stated that light was not necessary for the observer effect, and that anything that could record which path the electron took would suffice to collapse the wave function. But in a later answer you said that light interferes with the electron. Which is it? Does every measuring device ("observer") require the addition of energy to the system (in the form of light or otherwise) that may merely be having a physical effect on the electron such that observing how the wave pattern is made is impossible? Or have experiments been done that control for this possibility, but in which observation nonetheless collapses the wave function? I find that virtually every author is ambiguous on this point.- Peter (age 54)Boston, MA, USA

A:

The key question is whether the different quantum possibilities lead to different large-scale outcomes. For example, the spin-up part of a state might lead to a bell ringing and the spin-down part not lead to that. These situations where different parts of the quantum state lead to diffeent large-scale effects are called "measurements". Any sort of coupling between the little quantum system and the large-scale world might be involved in a measurement. Electromagnetism, including light, is the most common coupling in our experience simply because most of the forces we're familiar with boil down to electromagnetism.

I don't think that every measuring device requires the addition of energy to the small quantum system. In fact, some measurements destroy the little system. For example, an ordinary photodetector destroys the photon it measures. So in that case the energy transfer is from the little system (photon) to the big one (photodetector).